Thermal-mechanical signal processing

ABSTRACT

A source signal is converted into a time-variant temperature field with transduction into mechanical motion. In one embodiment, the conversion of a source signal into the time-variant temperature field is provided by utilizing a micro-fabricated fast response, bolometer-type radio frequency power meter. A resonant-type micromechanical thermal actuator may be utilized for temperature read-out and demodulation.

RELATED APPLICATION

This application is a Continuation Under 35 U.S.C. § 1.111 (a) ofInternational Application No. PCT/US2004/027229, filed Aug. 20, 2004 andpublished in English as WO 2005/020482 A3 on Mar. 3, 2005, which claimspriority to U.S. Provisional Application Ser. No. 60/496,431 (entitledMethod and Apparatus for Thermal-Mechanical Signal Processing, filedAug. 20, 2003), which applications are incorporated herein by reference.This application also claims priority to U.S. Provisional ApplicationSer. No. 60/496,421 (entitled Shell-Type Micromechanical Actuator andResonator, filed Aug. 20, 2003), and U.S. Provisional Application Ser.No. 60/496,430 (entitled Laser Annealing For MEMS Device, filed Aug. 20,2003), which are incorporated herein by reference. This application isrelated to U.S. application Ser. No. 10/097,078 (entitled Heat PumpedParametric MEMS Device, filed Mar. 12, 2002), which is incorporatedherein by reference.

GOVERNMENT FUNDING

The invention described herein was made with U.S. Government supportunder Grant Number DMR0079992 awarded by the National ScienceFoundation. The United States Government has certain rights in theinvention.

BACKGROUND

Many different methods exist for performing signal processing. In oneform of signal processing, devices are used to separate carrier signalsand extract base-band information from signals, such as broadcastsignals. Prior devices consumed more power and space than desired, andwere not easy to implement on a single-chip transceiver.

SUMMARY

A source signal is converted into a time-variant temperature field whichis used to produce mechanical motion. In one embodiment, the conversionof a source signal into the time-variant temperature field is providedby utilizing a micro-fabricated fast response, bolometer-type radiofrequency power meter. A resonant-type micromechanical actuator may beutilized for temperature read-out.

In one embodiment, partial or full signal processing necessary forseparating a carrier, and extracting base-band information from abroadcast signal is accomplished. Thermal-mechanical processing is usedfor frequency reference generation and frequency modulation of thecarrier signal by a base-band signal in a transmitter path. Powersavings and the ability to implement such signal processing on asingle-chip transceiver may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a signal processing device for convertingheat into motion according to an example embodiment.

FIG. 2 is a block diagram showing an oscillating disc with a bolometerfor converting heat into motion according to an example embodiment.

FIG. 3 is a block diagram showing an FM receiver using a microheater andresonator according to an example embodiment.

FIG. 4 is a block diagram of a frequency generator having a heatconverting feedback loop according to an example embodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description is, therefore, not to betaken in a limited sense, and the scope of the present invention isdefined by the appended claims.

FIG. 1 is block diagram illustrating an example embodiment of a system100 for converting a signal into mechanical motion. A bolometer 110,such as a micron or nanometer scale bolometer is used to receive RFsignals 115. In one embodiment an antenna converts the RFelectromagnetic signal into an electrical signal, which is thenconverted into heat, by the bolometer 110, such as by a small resistor.In further embodiments, the bolometer may be any type of device thatreceives RF electrical signals and converts it into heat. The bolometer110 in one embodiment has an antenna that converts the RF signal into anelectrical signal, which is then converted into heat, such as by a smallresistor. In further embodiments, the bolometer may be any type ofdevice that receives RF and converts it into heat. The bolometer 110 inone embodiment has a very small heat capacity, and exhibitsheating/cooling rates well above the MHz frequency range. This enableshigh frequency thermal operation for RF signal processing. The bolometermay also be selected to provide mixing capabilities.

In order to detect the high frequency, small amplitude temperatureoscillations of the bolometer 110, a thermo-mechanical actuator at 120is used. In one embodiment, the bolometer 110 is placed in contact withthe actuator 120, such to provide local temperature variations and hencelocal thermomechanical stress on the actuator. This causes the actuator120 to exhibit distortions, resulting in detectable mechanical motion ordisplacement. Such motion is detected by a sensor 130. The motion isthus a function of the frequency of the signal that heats the bolometer110.

The sensitivity of the thermal-to-mechanical transduction or inductioncan be enhanced by utilizing the resonant properties of thethermo-mechanical actuator. Matching the frequency of the temperatureoscillations with a resonant frequency of the actuator provides anincrease in the amplitude of the resulting mechanical motion by a factorQ, which describes the quality factor of the resonator. The resonantproperties of the thermal actuator may also be employed to filter outunwanted frequency components of the incoming RF signal 115.

FIG. 2A is a side view representation of a micromechanical oscillator200 that serves as an actuator 120 of FIG. 1. FIG. 2B is a top view. Theoscillator 200 oscillates in the radio frequency (RF) range and isfabricated in the form of a concave membrane 210, which is generallycircular or disc shape, and has a hole 215 proximate the center. Themembrane is clamped on the periphery to the surrounding polysiliconfilm. Other types of resonators may be used, such as discs supported bypillars, cantilevered beams, and other devices.

Oscillator 200 may be fabricated by growing an approximately 1 um layerof silicon oxide 250 on a surface of silicon wafer, or similar substrate220. The oxide is used as a sacrificial layer. Polycrystalline siliconfilm 255 is then deposited on the surface of the oxide 250 by lowpressure chemical vapor deposition (LPCVD) at 590° C. After thedeposition, the wafer is annealed for approximately 15 minutes atapproximately 1050° C. Photolithography, followed by a CF4 dry etch isused to create an approximately 2 um-diameter hole 215 through the toppolysilicon layer 225.

After stripping off resist used in the lithography process, thestructure is dipped into concentrated hydrofluoric acid (HF 49%).Dissolving the sacrificial silicon dioxide (etch rate approximately 1um/minute) results in a suspended membrane-like structure with a hole inthe center. The etching time determines the outer diameter of the cavity260 underneath the polysilicon film. This is provided as one example ofa method of making such an oscillator. As will be apparent to one ofaverage skill, many of the parameters and materials may be variedsignificantly to make dome shaped oscillators.

If the polysilicon were stress-free, the released membrane would beflat. However, significant compressive stress incorporated in thepolysilicon film as a result of deposition and annealing parametersmakes the planar configuration unstable and leads to a buckled membrane.The resulting structure has a dome shape with a hold at the top. Using acritical point dry (CPD) process to avoid surface tension provides ahigh yield for the dome-type resonators, and prevents stiction of themembrane to the substrate.

A modified method of fabrication of the shell membrane 210 begins with aCF₄ plasma dry etch of the self aligned apex hole 215 in the devicelayer. The opened hole provides a way to dissolve an underlying 1.5 μmsilicon dioxide layer 250 with hydrofluoric acid, creating a suspendedmembrane above the substrate. The resulting membrane 210 has non-zerocurvature due to large compressive stress incorporated into the devicelayer through high temperature post process annealing. When thesacrificial silicon dioxide is released through the etch hole, thein-plane stress produces out of plane buckling, forming a shallow shell.The resonator used in the present study is a 200 nm thick polysiliconshallow spherical shell segment, 30 μm in diameter, projectingapproximately 1 μm out of plane at the apex.

The out-of-plane component in the dome-like membrane significantlyincreases the resonant frequency several fold over a two dimensionalstructure. The natural frequency of a flat annulus clamped on theperiphery and free in the center is described by $\begin{matrix}{f_{mn}{_{\underset{\underset{plate}{annular}}{2D}}{= {\frac{\pi\quad h}{2R^{2}}\sqrt{\frac{E}{3\quad\rho\quad\left( {1 - v^{2}} \right)}}\left( \beta_{mn} \right)^{2}}}}} & (1)\end{matrix}$where h is the polysilicon thickness, R is the radial projection of theplate, E is Young's modulus, ρ is the material density, υ is Poisson'sRatio, and β is a geometrical constant.

Shallow shell theory is used to derive equation 2 which accounts for theextra rigidity of the out of plain projection $\begin{matrix}{f_{mn}{_{\underset{\underset{shell}{spherical}}{shallow}}{= \left( {f_{mn}^{2}\left. _{\underset{plate}{flat}}{+ \frac{E}{\rho\quad\left( {2\quad\pi\quad\chi} \right)^{2}}} \right)^{1/2}} \right.}}} & (2)\end{matrix}$

where χ is the radius of curvature of the dome. The increased stiffnessallows large radial dimension structures to achieve significantly higherfrequencies. In particular, the aforementioned 30 μm diameter dome maydisplay a 17.8 MHz γ₁₁ resonance with a quality factor of ˜10,000.Numerical modeling suggests that deeper shells with smaller lateraldimensions have the potential to reach into the GHz range. Operation inair may occur; however, viscous damping forces over the large surfacearea reduce the Q to ˜70.

A bolometer 240 or other type of resistive heating element is placed onthe membrane 210 near the periphery in one embodiment. It may be placedin any position on a membrane that will result in a modulation of thevibration of the membrane in response to heating and cooling of thebolometer 240.

Transduction between electrical signals and mechanical motion isaccomplished by creating a non-homogeneous thermal mechanical stress inthe dome structure by means of the resistive heating element 240dissipating Joule heat into the oscillator. The high rigidity of thedome structure allows use of a second layer of lithography to create ametallic resistor on the top of the prefabricated shell resonator. Inone embodiment optical lithography is performed on the polysilicon,followed by image reversal in a YES oven. An electron gun evaporator isused to deposit a 5 nm titanium adhesion layer and a subsequent 20 nmfilm of gold on the polysilicon surface. Liftoff is performed with anacetone soak and IPA rinse. Since the presence of even a thin metalliclayer increases dissipation of elastic energy in the moving structure,the quality factor of the resonator may be tailored by placing theheater at locations with different displacement amplitudes. At the sametime, matching of the input impedance to that of the correspondingcircuitry can be achieved by changing the geometry of the resistor. Inone embodiment, the bolometer comprises a 50Ω, 70×3 μm resistor 240 onthe periphery of the dome, allowing the resonator to be undamped by the2^(nd) layer metallization.

When a 20 mV signal is applied to the resistor, approximately 4 μW ofJoule heat may be dissipated into the resonator. Adsorbed heat in theresonator produces thermal stress in the polysilicon device layer, and,due to coupling between the membrane and flexural components within theshell structure, this thermal stress produces significant out-of-planedeflections within the resonator. The fact that bending of the resonatoris facilitated through the curved structure (rather than throughdiffering expansion coefficients in multiple layers) reduces dampinginduced by lossy layers observed in bimorph resonators.

The process of heat diffusion within the resonator can be modeled by aone dimensional heat equation, $\begin{matrix}{{u_{n}\left( {r,t} \right)} = {B_{n}\sin\quad\left( \frac{n\quad\pi\quad r}{R} \right){\mathbb{e}}^{{- \lambda_{n}}t}}} & (3)\end{matrix}$with time constant, $\begin{matrix}{\lambda_{n} = {\frac{K}{C\quad\rho}\left( \frac{{n\quad\pi}\quad}{R} \right)^{2}}} & (4)\end{matrix}$where K is the thermal conductivity, C is the heat capacity, R is theradius of a 2D plate, and n=2.4 is the root of a Bessel functionJ₀(μ_(i) ⁽⁰⁾)=0. Due to the micron size radius of the device, thecooling rate, 1/λ, is on the order of microseconds, allowing highfrequency modulation of the dissipated power through application of anAC signal to the resistor. When the frequency of the applied signalmatches the natural frequency of the mechanical vibrations, f₀, providedthat 1/λ<1/f_(o), detectable high amplitude vibration occurs.

The driving force, which is proportional to the power dissipated in theresistor, is, after removing 2f components:F_(drive)∝V_(DC) sin(ω_(o)t)  (5)In the 30 μm domes, sufficient mechanical displacement may be obtainedwith a 20 mV AC signal, reducing power consumption of the transducer.Post production resonator tuning is possible by controlling the DCvoltage level through the resistor. The 30 μm dome showed a (2 Hz/μW)dependence on the applied DC bias.

The vertical displacement of the vibrating shell is detected opticallyby measuring reflectivity of a 632.8 nm HeNe laser beam focused on thedevice in a 10⁻⁶ Torr vacuum chamber. The modulation of the reflectedlight, introduced by a moving Fabry-Perot interferometer (created by thesubstrate and the suspended structure) was detected by a New Focus 1601high-speed photodetector and analyzed in an Agilent 4396Bspectrum/network analyzer.

FIG. 3 is a block diagram of an FM radio receiver 300. An amplifiedsignal received via and antenna 310 is linearly superimposed at 320 witha reference signal from a local oscillator 325. This combined signal isprovided to a device 330 formed of a shell or dome type disc with amicroheater formed on a downslope or periphery of the disc. The combinedsignal is used to modulate the temperature of the microheater 332, whichin one embodiment comprises a metal resistive element that dissipatesheat in response to electrical current.

In one embodiment, the amplified signal received via antenna 310 isamplified by 40 dB, and linearly superimposed with a 0 dB referencesignal from the local oscillator 325. The local oscillator 325 is tunedto 87.2 MHz so that the frequency difference between a station ofinterest (97.3 MHz for example) would match a resonant frequency of thedome, (f_(resonator)=10.1 MHz). The dome resonator passes thedown-converted signal within the f_(resonator)/Q band, filtering out allunwanted carrier frequencies. Due to high quality factors (Q˜3,000) ofthe dome, only a fraction (δf_(FM)˜3 kHz) of the FM modulated (δf ˜100kHz) signal passes through the mechanical filter. By positioning theupper or lower sloping skirt of the FM band at the dome resonantfrequency, a slope detection method is provided to demodulate the FMcarrier. The amplitude of the mechanical vibration, used as the outputsignal, becomes entangled with the frequency detuning of the FM signaland a simple envelope detector 335 is used to convert the resultingsignal into audio 340. In one embodiment, the envelope detectorcomprises an interferometer focused on the disc. In further embodimentscapacitive detection or piezoelectric detection methods may be employed.

FIG. 4 is a block diagram of a frequency generator 400. The frequencygenerator comprises a disc 410, such as a shell or dome, that vibratesat a selected desirable frequency. A heating/cooling element, such as aresistive bolometer 420 is thermally coupled to an edge or other desiredportion of the disc 410 to cause the oscillation of the disc in responseto temperature changes. A detector 430 provides an output signal 435responsive to such oscillation of the disc. Output signal 435 isamplified, phase shifted and fed back along a feedback line 440 to thebolometer 420, providing a positive feedback loop. The disc becomesself-oscillating at a desired frequency. A self-sustained oscillation ofthe dome resonator is obtained with frequency stability of 1.5 ppm.

The bolometer and actuator may also serve as a RF signal demodulator. Inother words, thermally driven MEMS resonators can simulate developmentof new methods of modulation applied to a carrier signal for informationbroadcasting. In one configuration, RF amplitude-modulated (AM) signalsare applied directly to the micro-fabricated resistive heater orbolometer. If the thermal inertia of the bolometer is small enough(1/f_(carrier)<<τ_(cool)<<1/f_(modul)), the temperature of the bolometerwill follow the modulation of the RF power (i.e. AM) while beingunaffected by the carrier frequency and thus, carry all the base bandinformation.

Another method of thermal signal processing, mimicking a superheterodyneRF receiver, is achieved by employing an intrinsic nonlinearity of theresistive bolometer. The temperature modulation is determined by thepower of the RF signal and hence by the square of the applied voltage.When a superposition of AC voltages at different frequencies is appliedto the ohmic heater, the resulting thermal oscillations appear atfrequencies equal to linear combinations of the input signals. Thisallows the resistive bolometer to perform as a frequency mixer:$\begin{matrix}{{{\Delta\quad T_{BOLOMETER}} \sim P_{DISSIPATED}} = \left( {{V_{1}\sin\quad{t\left( {\omega_{1}t} \right)}} + {V_{2}\sin\quad\left( {\omega_{2}t} \right)}} \right)^{2}} \\{= {{V_{1}^{2}/2} - {{V_{1}^{2}/2}*\cos\quad\left( {2\quad\omega_{1}} \right)} +}} \\{{{V_{1}V_{2}\cos\quad\left( {\omega_{1} - \omega_{2}} \right)} - {V_{1}V_{2}\cos}}\quad} \\{{\left( {\omega_{1} - \omega_{2}} \right)t} - {{1/2}*V_{2}^{2}\cos\quad\left( {2\quad\omega_{2}t} \right)} +} \\{V_{2}^{2}/2}\end{matrix}$

Components at f_(up)=ω₁+ω₂ and f_(down)=ω₁-ω₂ are referred to as “up”and “down” converted signals. Applying a combination of the signal ofinterest at ω₁ and the signal from a local oscillator at ω₂ to thebolometer and choosing the thermal intertia of the bolometer as1/f_(up)<<τ_(cool)<<1/f_(down), thermal oscillations only at thefrequency f_(down) are produced, an analog to an intermediate frequency(IF) in a superheterodyne receiver.

In order to detect the down-converted component of the RF signal, smallamplitude temperature oscillations of the bolometer are converted intomechanical motion by using a micro-fabricated thermo-mechanicalactuator, such as a shell or dome oscillator. In one embodiment, thebolometer or resistive heater is placed on a slope of a micro-fabricateddome-type structure. RF signals are applied to it. Local variations ofthe dome result in detectable mechanical motion, such as by aninterferometer.

The sensitivity of the thermal-to-mechanical transduction can beenhanced greatly if resonant properties of the thermo-mechanicalactuator are employed. By matching the frequency of the temperatureoscillations with a resonant frequency of the actuator, the amplitude ofthe resulting motion is amplified by the factor Q, which describes thequality factor of the resonator. The resonant properties of the thermalactuator are also employed to filter out unwanted frequency componentsof the incoming RF signal. Parametric amplification may also be employedby pumping the actuator at twice its resonant frequency.

A time dependent envelope of the mechanical vibration amplituderepresents the base-band information for both amplitude modulated (AM)and frequency modulated (FM) signals. For FM signals, the width of theresonant peak of the thermo-mechanical actuator is narrower orcomparable to the band of the FM broadcast. Variation of the modulationfrequency shifts the frequency of the thermal oscillations within themechanical resonant peak and hence affects the amplitude of themechanical vibrations. Thus, the thermo-mechanical actuator-resonatorcan perform as a combination of a filter (IF filter in particular) anddemodulator. After converting the mechanical motion of the actuator intoan electrical signal, it can be applied to a hearing device or base-bandelectronics for further processing. The radio receiver 300 is apractical example of an FM radio receiver.

In a further example embodiment, dome-shaped, radio frequency,micromechanical resonators are with integrated thermo-elastic actuators.Such resonators can be used as the frequency-determining element of alocal oscillator or as a combination of a mixer and IF filter in asuperheterodyne transceiver.

CONCLUSION

A MEMS-based method and apparatus may be used to implement mixing,filtering, demodulation, frequency reference and frequency modulation. Amethod and apparatus are provided by converting a source signal into atime-variant temperature field with consequent transduction intomechanical motion. In one embodiment, the first conversion (signal intotemperature) is provided by utilizing a micro-fabricated fast response,bolometer type radio frequency power meter. A resonant typemicromechanical thermal actuator is utilized for temperature read-out,filtering and demodulation.

The method may be applied to a wireless communication device, and canaccomplish partially or in full, the signal processing necessary forseparating the carrier and extracting base-band information from thebroadcast signal. Similar thermal mechanical processing can be used forfrequency reference generation and frequency modulation of the carriersignal by a base-band signal in the transmitter path. The primarybenefits are savings in power consumption, reduced size of componentsand single-chip transceiver solutions.

Converting an RF signal into heat and measuring the resultingtemperature rise constitute a basic bolometric method for measuring RFpower. Micron-size and nano-scale bolometers provide heating/coolingrates well above MHz frequency range and enable high frequency thermaloperation for RF signal processing.

A heat actuated mixer and mechanical resonator device may be implementedin a wireless communicating device that eliminates the necessity ofbulky off-chip components, and shrinks dimensions drastically. A lowpower PHEAT-30 uW for 10 MHz dome-type device) and low voltages used tooperate the device may be very beneficial in such microminiaturizedwireless communicating devices. The low impedance of the ohmic drive maysimplify RF matching considerably.

In one embodiment, the dome resonators, comprise shallow shell segmentsclamped on the periphery. They may be fabricated utilizing pre-stressedthin polysilicon film over sacrificial silicon dioxide. The shellgeometry enhances the rigidity of the structure, providing a resonantfrequency several times higher than a flat membrane of the samedimensions. The finite curvature of the shell also couples out-of-planedeflection with in-plane stress, providing an actuation mechanism.Out-of-plane motion is induced by employing non-homogeneous,thermomechanical stress, generated in plane by local heating. A metalresistor, lithographically defined on the surface of the dome, providesthermal stress by dissipating heat. The diminished heat capacity of theMEMS device enables a heating/cooling rate comparable to the frequencyof mechanical resonance and allows operation of the resonator byapplying AC current through the microheater. Resistive actuation can bereadily incorporated into integrated circuit processing and providessignificant advantages over traditional electrostatic actuation, such aslow driving voltages, matched impedance, and reduced cross talk betweendrive and detection.

The shell type resonator is but one example of a resonator fortemperature readout. Example resonators that will couple to the thermalactuator may be in the form of a cantilever, disk supported by a pillaror a clamped beam. Still further resonators will be apparent to those ofskill in the art after reading the detailed description.

1. A method of signal processing for extracting base-band informationfrom a broadcast signal, the method comprising: converting a radiofrequency signal into heat; and measuring temperature changesrepresentative of the radio frequency signal.
 2. A method of signalprocessing for extracting base-band information from an amplitudemodulated broadcast signal, the method comprising: converting theamplitude modulated broadcast signal into heat using a micro-fabricatedresistive heater having a thermal inertia small enough to follow themodulation; and measuring temperature changes in the micro-fabricatedresistive heater.
 3. The method of claim 2 wherein oscillations in theheat are converted into mechanical motion.
 4. The method of claim 3wherein a micro-fabricated thermo-mechanical actuator is used for themechanical motion.
 5. The method of claim 4 wherein the actuator has aresonant frequency that matches the frequency of the temperaturechanges.
 6. A device for extracting base-band information from abroadcast signal, the device comprising: means for converting thebroadcast signal into temperature oscillations; and means for convertingthe temperature oscillations into mechanical motion.
 7. A device forextracting base-band information from a broadcast signal, the devicecomprising: a micro-fabricated bolometer that converts the broadcastsignal into temperature oscillations; and a micro-fabricatedthermo-mechanical actuator proximate the bolometer that converts thetemperature oscillations into mechanical motion.
 8. The device of claim7 wherein the actuator has a resonant frequency that approximatelymatches the frequency of temperature oscillations.
 9. The device ofclaim 7 wherein the bolometer is directly coupled to a peripheralportion of the actuator.
 10. The device of claim 7 wherein the actuatorcomprises a dome type micro-fabricated oscillator.
 11. The device ofclaim 10, wherein the dome type micro-fabricated actuator comprises ahole in the center, and is supported by a substrate.
 12. The device ofclaim 11, wherein the substrate further comprises circuitry forprocessing the base-band information.
 13. A method comprising:converting a source signal into a time-variant temperature field;modulating mechanical motion with the time-variant temperature field;and measuring the mechanical motion.
 14. The method of claim 13 whereinthe measured mechanical motion is converted into an output signalrepresentative of the source signal.
 15. The method of claim 13 whereinthe mechanical motion is provided by a mechanical resonator having aresonant frequency compatible with the source signal.
 16. The method ofclaim 15 wherein the mechanical resonator has dimensions in themicrometer to nanometer range.
 17. A device for extracting base-bandinformation from a broadcast signal, the device comprising: amicro-fabricated bolometer that converts the base-band information inthe broadcast signal into a time varying heat field; a micro-fabricatedthermo-mechanical actuator having a dome shape supported by a substrate,the bolometer positioned on a periphery of the dome shape such that thetime varying heat field provided by the bolometer modifies mechanicaloscillation of the actuator; and means for generating a signalresponsive to the modified mechanical oscillation of the actuator. 18.The device of claim 17 wherein the means for generating a signalresponsive to the modified mechanical oscillation of the actuatorcomprises an interferometer.
 19. A frequency generator comprising: amechanical resonator; a microheater coupled to the resonator; means formeasuring motion of the mechanical resonator; and means for providingfeedback to the microheater.
 20. The device of claim 19 wherein theresonator comprises a dome type micro-fabricated oscillator.
 21. Thedevice of claim 20, wherein the dome type micro-fabricated actuatorcomprises a hole in the center, and is supported by a substrate.
 22. Thedevice of claim 19 wherein the microheater comprises a bolometer that isthermally responsive to RF signals.
 23. The device of claim 19 whereinthe means for measuring motion of the mechanical resonator comprises aninterferometer.